Apparatus and a method for launching plugs

ABSTRACT

The invention relates to an apparatus and a method for launching plugs used in the construction of an oil or of gas well. There is provided an apparatus for launching a plug, said apparatus comprising a canister ( 4 ) connectable to a running string, a plug ( 6, 16 ) retained in said canister, and a tube ( 9, 13 ) having a bursting disk ( 37, 38 ), the arrangement being such that, in use, an increase in fluid pressure in said tube ruptures said bursting disk allowing an increase in fluid pressure above said plug to launch said plug. The apparatus is particularly suited to launching two, three or more plugs independently. There is also provided a method of launching a plug from a canister, which method comprises the steps of blocking a tube; increasing pressure in said tube above said blockage to a predetermined pressure at which a bursting disk in said tube ruptures allowing fluid pressure to increase above said plug and launches said plug.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to an apparatus and a method for launching plugsused in the construction of an oil or gas well.

In the construction of an oil or gas well, a hole is drilled in theground. A casing string is then lowered into the ground and hung fromthe surface. The casing string is then cemented into the wellbore. Thecement is pumped down through the casing string and forced out of thebottom of the casing string through a float shoe and a float collar andup the annulus formed between the external surface of the casing stringand the internal wall of the wellbore. A first plug is usually launchedfrom the top of the string ahead of the cement. A second plug is usuallylaunched after the cement to act, amongst other things, as a barrierbetween the cement and the fluid being used to force the cement down thecasing, and also to wipe clean the inside of the casing.

In the preparation of certain wells, a hole is drilled through theground below a casing string. A liner of smaller diameter than thecasing is then lowered through the casing and hung from the bottom endof the casing string. The liner is then cemented into the wellbore. Thecement is pumped down through the liner and forced out of the bottom ofthe liner through a float shoe and float collar and up the annulusformed below the external surface of the liner and the internal wall ofthe wellbore or of the internal surface of the casing. A first plug isusually launched from the top of the liner ahead of the cement. Atsecond plug is usually launched after the cement to act, amongst otherthings, as a barrier between the cement and the fluid being used toforce the cement down the liner and also to wipe clean the inside of thecasing.

One of the problems of existing apparatus is ensuring that plugs arelaunched reliably.

A bursting disk is disclosed in FIG. 27 of Applicants' PCT PublicationNo. WO 96/34175. However, this is used to facilitate the separation oftwo plugs which may have been accidentally launched together.

The invention attempts to solve this problem.

Accordingly there is provided an apparatus for launching a plug, theapparatus comprising a canister connectable to a running string, a plugretained in. said canister, and a tube having a bursting disk, thearrangement being such that, in use, an increase in fluid pressure insaid tube ruptures said bursting disk allowing an increase in fluidpressure above said plug to launch said plug.

Preferably, the apparatus further comprises a landing seat in said tubearranged below said bursting disk, the arrangement being such that, inuse, when a dart lands on said landing seat, pressure is increased abovesaid dart to rupture said bursting disk allowing an increase in fluidpressure above said plug to launch said plug.

Advantageously, the tube has an upper and a lower end, said lower endbeing attached to said plug and said upper end being slidably arrangedwith respect to a second tube.

Preferably, there is a sealing ring arranged between said tube and saidsecond tube.

Advantageously, there is provided a further bursting disk in said tubearranged below said landing seat. The further bursting disk is designedto burst at a higher pressure than the bursting disk. The furtherbursting disk ruptures after the plug lands on a float collar, floatshoe or other obstacle in the casing or liner and after a considerablepressure has been applied. This would then allow cement to flow throughthe plug and into the annulus between the casing or liner and thewellbore or casing.

Preferably, said tube has a portion which extends from said landing seatto retain said dart.

Advantageously, at least one port is provided in said canister abovesaid plug. The port aids smooth ejection of the plug.

Preferably, said tube is substantially concentric with said plug andthere is provided an annulus therebetween. The annulus should not becontinuous along the length of the plug.

If so desired the apparatus may comprise a second plug which is attachedto said second tube, said second tube having a second bursting disk andbeing slidable with respect to a third tube.

Advantageously, the apparatus further comprises a second landing seat insaid second tube arranged below said second bursting disk, thearrangement being such that, in use, when a second dart lands on saidsecond landing seat, pressure is increased above said second dart torupture said second bursting disk allowing an increase in fluid pressureabove said second plug to launch said second plug.

Preferably, a sealing ring is arranged between said second tube and saidthird tube.

Advantageously, there is provided a further bursting disk in said secondtube arranged below said second landing seat.

Preferably, at least one port is provided in said canister above saidsecond plug. The ports may be placed between the plugs to aid separationthereof.

Advantageously, said second tube has a portion which extends from saidsecond landing seat to retain said second dart.

Preferably, said second tube is substantially concentric with saidsecond plug and there is provided an annulus therebetween.

If so desired the apparatus further comprises a third plug which isattached to said third tube, said third tube having a third burstingdisk and which third tube is slidable with respect to a fourth tube.

Advantageously, the apparatus further comprises a third landing seat insaid third tube arranged below said third bursting disk, the arrangementbeing such that, in use, when a third dart lands on said third landingseat, pressure is increased above said third dart to rupture said thirdbursting disk allowing an increase in fluid pressure above said thirdplug to launch said third plug.

Preferably, a sealing ring arranged between said third tube and saidfourth tube.

Advantageously, there is provided a further bursting disk in said tubearranged below said third landing seat.

Preferably, least one port is provided in said canister above said thirdplug.

Advantageously, said third tube has a portion which extends from saidthird landing seat to retain said third dart.

Preferably, said third tube is substantially concentric with said thirdplug and there is provided, an annulus therebetween.

Advantageously, said second tube is shear pinned to said third tube in atwo plug arrangement.

Preferably, said third tube is shear pinned to said fourth tube in athree plug arrangement.

Advantageously, said canister comprises at least one retaining rib foreach of said plugs.

Preferably, each of said plugs comprises fins and wherein said canistercomprises at least one retaining rib for each of said fins.

There is also provided a method of launching a plug from a canister,which method comprises the steps of blocking a tube; increasing pressurein said tube above said blockage to a predetermined pressure at which abursting disk in said tithe ruptures allowing fluid pressure to increaseabove said plug and launches said plug.

In a two or more plug arrangement the method comprises the further stepof increasing pressure between said plug and second plug launching saidplug.

In a three or more plug arrangement the method comprises the furtherstep of increasing pressure between said second plug and third pluglaunching said second plug.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention reference will now be made,by way of example, to the accompanying drawing which shows alongitudinal cross-sectional view of an apparatus in accordance with thepresent invention.

Referring to the drawing there is shown a plug launching apparatus whichis generally designated by reference number 1 in a section of a casingstring 2. The plug launching apparatus 1 is connected to a runningstring (not shown) by an adaptor 3.

The apparatus 1 comprises a canister 4 having a cylindrical portion anda hub 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A first plug 6 having four fins 8 is retained in the canister 4 by aninternal rib 7. Only one rib 7 is shown, however, it is envisaged thatribs will be placed below each fin 8. The first plug 6 is provided witha substantially concentric first tube 9 which is glued or otherwiseattached to the bottom 10 of the first plug 6. An annulus is providedbetween the first tube 9 and the first plug 6 above the bottom 10. Thefirst tube 9 is provided with a first dart landing seat 11 above whichis located a bursting disk 12 in the first tube 9. The top of the firsttube 9 is slidably arranged in a second tube 13. A sealing ring 14 isprovided between the first tube 9 and the second tube 13. An annulus isprovided around the first bursting disk 12 and a second plug 16.

In use, a first dart (not shown) is pumped down the running string. Thefirst dart will land on the first dart landing seat 11. Pressure israised in the running string above the first dart to a firstpredetermined pressure, at which the first bursting disk 12 rupturesallowing a pressure increase above the first plug 6 and below a secondplug 16, the plug 6 launches taking with it the first tithe 1 and thefirst dart (not shown) which is at least at least partly retained insaid first tube 9. It should be noted that before the at least one firstbursting disk 12 ruptures, there is an area across the first dartsubjected to raised pressure, however the resultant force is not greatenough to launch the first plug.

The second plug 16 is retained in the canister by a rib 17. Only one rib17 is shown, however, it is envisaged that ribs will be placed beloweach fin 18. The second plug 16 is provided with a substantiallyconcentric second tube 13 which is glued or otherwise attached to themiddle 20 of the second plug 16. An annulus is provided between thesecond tube 13 and the second plug 16 above and below the middle 20. Thesecond tithe 13 is provided with a second dart landing seat 21, whichhas a larger opening than that of the first dart landing seat 11. Abovethe second landing seat 21 there is located at least one second burstingdisk 22 in the second tube 13. The top of the second tube 13 is slidablyarranged in a third tube 13. A sealing ring 24 is provided between thesecond tube 13 and the third tube 23. An annulus is provided around thesecond bursting disk 22 and the third plug 26.

In use, a second dart (not shown), the nose of which would be of largerdiameter than the first dart, is pumped down the running string. Thesecond dart will land on the second dart landing seat 21. Pressure israised in the running string above the second dart to a secondpredetermined pressure, greater than the first predetermined pressure,at which the second bursting disk 22 ruptures allowing a pressureincrease above the second plug 16 and below the third plug 26. Thesecond plug 16 launches, taking with it the second tube 13 and thesecond dart (not shown) which is retained in said second tube 13. Itshould be noted that before the at least one second bursting disk 22ruptures, there is an area across the second dart subjected to raisedpressure, however the resultant force is not great enough to launch thesecond plug.

The third plug 26 is retained in the canister by a rib 27. Only one ribis shown, however, it is envisaged that ribs will be placed below eachfin 28. The third plug 26 is provided with a substantially concentricthird tube 23 which is glued or otherwise attached to the top 30 of thethird plug 26. An annulus is provided between the third tithe 23 and thethird plug 26 below the top 30. The third plug is provided with a thirddart landing seat 31, which has a larger opening than that of the seconddart landing seat 21. Above third dart launching seat 31 there islocated at least one third bursting disk in the third tube 23. The topof the third tube 23 is shear pinned to hub 5 by at least one shear pin34.

In use, a third dart (not shown), the nose of which would be of greaterdiameter than the second dart, is pumped down the running string. Thethird dart will land on the third dart landing seat 31. Pressure israised in the running string above the third dart to a predeterminedpressure, at which at least one shear pin 34 shears. The plug 26launches, taking with it the third tithe 23 and the third dart (notshown) which is at least partly retained in third tube 23.

Further bursting disks 37, 38, 39 are provided in the first, second andthird tubes 9, 13, 23 so that at predetermined pressures, the furtherbursting disks can rupture and fluid may pass through the first, secondand third plugs 6, 16, 26 after launching and landing thereof.

Small holes 40 and 41 in the canister 4 act as dampeners allowing a slowpressure equalisation and aid smooth release of each plug 6, 16, 26independently.

It should be noted that the clearance between the casing string 1 andthe canister 4 may be as small, as possible.

The above described apparatus could be modified to launch four or moreplugs independently of one another as well as one, two or three.

Various modifications to the embodiment described are envisaged, forexample the ribs could be dispensed with, reliance being placed on theradial force between the plugs and the canister. However, this is notrecommended since it is much preferred to very firmly retain the plugsin the canister and to rely on substantially hydraulic pressure to ejectthe plugs.

What is claimed is:
 1. An apparatus for launching a plug, comprising: acanister connected to a running string; a plug retained in the canisterand; a tube disposed within the plug wherein the tube further comprisesa landing seat to receive a dart and a bursting disk disposed in thetube above the landing seat such that when said bursting disk rupturesfluid pressure within the drill string is then applied to the top of theplug to launch the plug.
 2. An apparatus for launching a plug accordingto claim 1 further comprising a further bursting disk disposed withinthe tube below the landing seat.
 3. An apparatus for launching a plugaccording to claim 2 wherein the rupture disk and the further rupturedisk are disposed in a sidewall of the tube.
 4. An apparatus forlaunching a plug according to claim 2 wherein the further rupture diskruptures at a higher pressure than the rupture disk.
 5. An apparatus forlaunching a plug according to claim 1 wherein the tube is substantiallyconcentric within the plug.
 6. An apparatus for launching a plugaccording to claim 1 wherein the plug further comprises a fin and thecanister further comprises a rib wherein the rib is positioned beneaththe fin so as to retain the plug within the canister.
 7. An apparatusfor launching a plug, comprising: a canister connected to a runningstring; a first plug retained in the canister and; a first tube disposedwithin the plug, the first tube further comprising a first landing seatand a first bursting disk disposed in a wall of the first tube above thefirst landing seat; a second plug retained in the canister above thefirst plug and; a second tube disposed within the second plug, thesecond tube further comprising a second landing seat and a secondbursting disk disposed in a wall of the second tube above the secondlanding seat wherein; when said first bursting disk ruptures fluidpressure is ported between the first plug and the second plug to launchthe first plug; and when said second bursting disk ruptures fluidpressure within the drill string is applied to the top of the secondplug to launch the second plug.
 8. An apparatus for launching a plugaccording to claim 7 wherein the first tube is slidably arranged withinthe second tube.
 9. An apparatus for launching a plug according to claim7 further comprising a sealing ring provided between the first tube andthe second tube.
 10. An apparatus for launching a plug according toclaim 7 wherein the first tube is substantially concentric within thefirst plug and the second tube is substantially concentric within thesecond plug.
 11. An apparatus for launching a plug according to claim 7wherein the second landing seat has a larger opening than said firstlanding seat.
 12. An apparatus for launching a plug according to claim 8further comprising a first further bursting disk disposed within thefirst tube below the first landing seat and a second further burstingdisk disposed within the second tube below the second landing seat. 13.An apparatus for launching a plug according to claim 12 wherein thefirst further bursting disk ruptures at a pressure greater than thepressure to rupture the first bursting disk and less than the pressureto rupture the second bursting disk and the second further burstingdisk.
 14. An apparatus for launching a plug according to claim 13wherein the second further bursting disk ruptures at a pressure greaterthan the pressure to rupture the second bursting disk.
 15. An apparatusfor launching a plug according to claim 13 further comprising: a thirdplug retained in the canister above the second plug and; a third tubedisposed within the third plug, the third tube further comprising athird landing seat and a third bursting disk disposed in a wall of thethird tube above the third landing seat wherein; and when said thirdbursting disk ruptures fluid pressure within the running string isported on top of the third plug to launch the third plug.
 16. Anapparatus for launching a plug according to claim 15 wherein the secondtube is slidably arranged within the third tube.
 17. An apparatus forlaunching a plug according to claim 15 further comprising a sealing ringbetween the second tube and the third tube.
 18. An apparatus forlaunching a plug according to claim 15 wherein the third tube issubstantially concentric within the third plug.
 19. An apparatus forlaunching a plug according to claim 15 wherein the third landing seathas a larger opening than the second landing seat.
 20. An apparatus forlaunching a plug according to claim 15 further comprising a thirdfurther bursting disk disposed within the third tube below the thirdlanding seat.
 21. An apparatus for launching a plug according to claim15 wherein the third further bursting disk ruptures at a pressuregreater than the pressure to rupture the third bursting disk.
 22. Amethod of launching a plug from a canister comprising the steps of:disposing a plug within a canister; disposing a tube within the plug;blocking the tube; increasing the pressure in the tube above theblockage to a predetermined pressure; rupturing a bursting disk withinthe tube; and launching the plug by applying fluid pressure to the topof the plug.
 23. A method according to claim 22 further comprising thesteps of: retaining a second plug within the canister and above thefirst plug; and launching the second plug by increasing the pressurebetween the plug and the second plug.
 24. A method according to claim 23further comprising the steps of: retaining a third plug within thecanister and above the second plug; and launching the third plug byincreasing the pressure between the second plug and the third plug. 25.A method of launching a plug, comprising: pumping a dart down a drillingstring and into a canister; landing the dart on a landing seat in a tubedisposed within a plug; increasing the pressure within the drillingstring to rupture a first bursting disk above the plug; and launchingthe plug by porting drill string pressure to the top of the plug.
 26. Amethod of launching a plug according to claim 25 further comprising:landing the plug on an obstacle; increasing the pressure within thedrill string to rupture a further burst disk; and flowing cement throughthe plug.
 27. A method of launching two plugs in a wellbore, comprising:pumping a first dart down a drilling string and into a canister; landingthe first dart on a landing seat in a first tube disposed in a firstplug and within the canister; increasing the pressure within the drillstring to rupture a bursting disk in the first tube; launching the firstplug by porting drill string fluid pressure to the top of the firstplug; landing a second dart on a landing seat in a second tube disposedin a second plug and within the canister; increasing the pressure withinthe drill string to rupture a bursting disk in the second tube; andlaunching the second plug by porting drill string fluid pressure to thetop of the second plug.
 28. A method of launching two plugs in awellbore according to claim 27 further comprising the steps of: landingthe first plug within the wellbore; rupturing a further bursting diskwithin the first tube; and pumping fluid through the first plug.
 29. Amethod of launching two plugs in a wellbore according to claim 28further comprising the steps of: landing the second plug within thewellbore; rupturing a further bursting disk within the second tube; andpumping fluid through the second plug.
 30. A method for launching threeplugs, comprising: disposing a first tube within a first plug; disposinga second tube within a second plug; disposing a third tube within athird plug; slidably arranging the first tube within the second tube andthe second tube within the third tube; retaining the first plug, thesecond plug and the third plug within a canister in a wellbore; landinga first dart in the first tube; increasing the pressure applied withinthe first tube to rupture a bursting disk within the first tube;launching the first plug by applying pressure to the top of the firstplug; landing a second dart in the second tube; increasing the pressureapplied within the second tube to rupture a bursting disk within thesecond tube; launching the second plug by applying pressure to the topof the second plug; landing a third dart in the third tube; increasingthe pressure applied within the third tube to rupture a bursting diskwithin the third tube; and launching the third plug by applying pressureto the top of the third plug.
 31. A method for launching three plugsaccording to claim 30 wherein fluid is pumped through the first plugcomprising: landing the first plug within a wellbore; increasing thepressure applied to the first tube to rupture a further bursting disk inthe first tube; and pumping a fluid through the first plug and furtherbursting disk.
 32. A method for launching three plugs according to claim30 wherein fluid is pumped through the second plug comprising: landingthe second plug within a wellbore; increasing the pressure applied tothe second tube to rupture a further bursting disk in the second tube;and pumping a fluid through the second plug and further bursting disk.33. A method for launching three plugs according to claim 30 whereinfluid is pumped through the third plug comprising: landing the thirdplug within a wellbore; increasing the pressure applied to the thirdtube to rupture a further bursting disk in the third tube; and pumping afluid through the third plug and further bursting disk.